1. Field of the Invention
The invention relates to a method for deter the baroreflex latent period, and particularly the corresponding baroreflex sensitivity.
2. Background Art
The test of baroreflex sensitivity (BRS) has long been a common method with which the function of the circulatory system is tested within the scope of clinical tests (see two list of prior art, No. [4]). For example, the test is used to quality the risk of patients having acute myocardial infarction (see list of prior art, No. [5]).
The BRS is the ratio of the resulting drop in the heart rate to an increase in the arterial blood pressure due to medication. The method is thus invasive, and cannot be performed without some discomfort for the patient. In addition, the application, similarly to Valsalva""s maneuver, or the decompression method (see list of prior art, No. [8]), which is controversial because of its low specificity, always requires outpatient treatment. A special evaluation procedure of the BRS test also permits conclusions about the latent period (see list of prior art, No. [6]), that is, the period between a drop in blood pressure and the response of the sino-atrial node due to the baroreflex but the procedure is far from conventional. A direct determination of the latent period from neural measurements as an exact method (see list of prior art, No. [7]) is highly invasive, and is therefore unsuitable for clinical diagnostics.
Recently, the baroreflex function has been indirectly incorporated through frequency analysis of the heart-rate variability, but this only encompasses the tonic partial aspect of the vagal activity (see list of prior art, No. [9]), or through the analysis of the heart-rate turbulence (see list of prior art, No. [10]). These two methods have a high prognostic value, but do not offer a direct measure for describing the baroreflex.
In contrast, in recent years, testing coupled systems in the field of non-linear dynamics has produced numerous methods and models for characterizing systems with an imminent time delay (see list of prior art, Nos. [11], [12]). Hence, it is also possible to reconstruct multi-dimensional systems by measuring a continuous variable (see list of prior art, No. [3]). Up to now, if the information about a system is present as interval data, however, only a partial reconstruction of the phase space has been possible, which can only be effected in strongly deterministic systems, with a sufficiently-long measuring period, in the stationary state (see list of prior art, No. [2]). Vital information about functional connections or an imminent time delay is not produced, however.
In addition, there are numerous models that simulate a heart-rate variability that is modulated by the baroreflex (see list of prior art, Nos. [13], [14]). One of these models is especially preferable, because it satisfies the requirement of factoring in the delay time of the baroreflex in a simple manner (see list of prior art, No. [1]). The model illustrates qualitatively-varying dynamic behavior, as a function of the time delay, with periodic, complex periodic and chaotic domains occurring.
Thus, two essential tools are available for testing new characterization methods of the baroreflex. On the one hand, it is possible to create a realistic, yet simple, model such that the dynamic processes triggered by changes in the latent period can be altered qualitatively to the greatest possible extent. On the other hand, methods of non-linear dynamics offer direct instruments for analyzing established model parameters exclusively from generated signals.
Starting from these set-ups, it is the object of the invention to permit a non-invasive, and therefore gentle, automatic testing and determination of the baroreflex latent period and, correspondingly, the baroreflex sensitivity, particularly using the carotid-sinus reflex.
According to the invention, in the method for determining the baroreflex latent period and, correspondingly, the baroreflex sensitivity, cardiac-interval periods, such as the P-P, P-R or R-R interval, are measured non-invasively, and the measured interval periods are mathematically analyzed according to non-linear dynamic methods.
The non-linear dynamic analysis method preferably comprises the following method steps:
the interpolation of the measured interval values for determining a continuous measured-value course from the discrete interval periods;
the graphing of the interpolated signal values in a three-dimensional phase space with the abscissas y(t), y(txe2x88x92xcfx84) and d(y)/d(t), with different baroreflex latent periods being assumed for the parameter xcfx84;
the projection of the three-dimensional graph into the two-dimensional space through the determination of all values on a sectional plane perpendicular to one of the axes, with y(t) preferably being constant; and
the determination of the assumed baroreflex latent period for which the highest order can be established in the two-dimensional projection as being relevant.
Checking whether the two-dimensional projection has an order is essentially the search for a line or course, as opposed to an xe2x80x9caggregate of points.xe2x80x9d For this purpose, the distances between points adjacent to a coordinate axis are added, with a minimal summation distance indicating a maximal order.
The proposed method is greatly simplified by the projection onto the two-dimensional space; the projection represents a type of scaling. A non-scalar, multi-dimensional set-up can also be advantageously employed, however, but such a set-up is associated with far more calculations.
The invention is described in detail below in conjunction with the attached drawings.